Some lead-acid storage battery manufacturers envelop the negative plates of their batteries in ion permeable, tree suppressing battery separator material made from sintered polyvinyl chloride (i.e., PVC) particles such as made by the process of U.S. Pat. No. 3,551,210, Feb. 3, 1969, Bahler et al. An elongated rectangular sheet of the PVC separator material is folded in the middle along the short dimension of the rectangle. The plate is inserted into the folded separator such that the bottom edge of the plate lies in the fold and the halves of the sheet on either side of the fold lay flat against the faces of the plate and overlie the lateral edges of the plate somewhat. Thereafter, the edges of the PVC separator material overlying the plate are cohesively bonded (i.e., heat sealed) to each other so as to form and open-topped envelope around the plate.
The sintered PVC battery separator material is relatively brittle and readily susceptible to damage (e.g., chipping, cracking, splitting, etc.) resulting from subsequent in-plant handling of the enveloped plates. This is particularly true at the fold. In this regard, the fold along the bottom edge of the plate is subjected to abuse each time the stack of plates is picked up and set down and particularly when it is dropped into the battery case during the "casing out" step.
Accordingly, it is an object of the present invention to toughen the folded edge of the separator envelope and thereby increase its ability to withstand in-plant handling. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.
In accordance with the invention, the folded edge of the separator is contacted with a plasticizer capable of wetting the separator and wicking up the sides thereof a short distance (i.e., about 0.3-0.6 cm) without excessively dissolving the PVC. N-methyl-2-pyrolidone is particularly useful for this application as well as combinations thereof with dimethyl and/or diethyl phthalates and water. The plasticizer preferably contains an inert dye which is used primarily to give visual indication of which separators, and the extent to which the separators, have been treated with the plasticizer.
Virtually any known plasticizer for PVC or combinations thereof can be useful with the present invention if it can be made to wet the separator without excessive dissolution thereof. In this regard, preferred plasticizers will have a sufficiently small molecule as to readily wet and wick into the sintered PVC and, is soluble in water so that its solvent strength can be readily controlled by dilution. In this regard, particular success has been achieved with n-methyl-2-pyrolidone, the properties of which may be used as a reference criteria for the selection of other plasticizers. This material has successfully been used in 85% by volume solution with water. Separators so treated have remained plasticized or toughened for periods of about 24 hours. If longer periods of toughening are required because of anticipated delays before final battery assembly, other plasticizers and particularly dimethyl and/or diethyl phthalate can be added to the n-methyl-2-pyrolidone. Such phthalate additions have provided separators adequately toughened for periods up to about five days.
In practicing the invention, a rectangular piece of PVC separator material is locally heated to soften the folding line and then folded along that line and around the bottom edge of the battery plate. The length of the piece is about twice the height of the plate. The fold is made in the center of the piece and the two half-pieces fold up to sandwich the plate therebetween. Thereafter the side edges of the piece adjacent the lateral edges of the plate are cohesively bonded together as by heat or ultrasonics. Following encapsulation, the fold is dipped (i.e., about one second) into a tray containing cellulose sponge saturated with the plasticizer. An appropriately colored (e.g., blue or red), but inert, dye may be added to the plasticizer (i.e., about 2 grams per five gallons of plasticizer) so as to provide sufficient coloring that ready detection of a plasticized separator is possible.